Not all paint is perfectly round, no matter what the manufacturer claims. Some are better and more consistant than others (cough valken cough), but even the best isn’t a perfect sphere. This leads to gaps when the ball is in the barrel. When there’s gaps, air escapes through those gaps and provides an uneven pressure on the ball, thus making the ball come out at an odd spin or angle, throwing off the accuracy of your shot.
This is where barrel sizing kits come in. By matching paint to the barrel you eliminate these air gaps, effectively ensuring more consistent pressure on the ball when you squeeze the trigger. A bonus is that because there is a better seal around the ball there’s no loss of pressure, making your ball velocity much more consistent.
To test ball to barrel fit, take a round and drop it in the back of your barrel. If it falls through, get a smaller barrel, fin or insert (depending on your kit). If it sticks, you should be able to blow the ball through with your own lung power. If you can’t the insert is too small, go up a size or two.
There are also some basic sizing tools you can pick up for a few dollars that have various holes for different sizes of paint. They’re a good investment, but after using a sizer, validate the choice by trying to blow a ball through the barrel/fin/insert as well.
Always remember that paintballs are highly susceptible to variances in the environment. If you’re at a big game or spend all day at the field, be sure to check your sizing periodically as the paint swells or shrinks with the weather for the best performance.